refers to motor weakness involving the arm and leg on the same side of
the body. Hemiparesis may occur with a lesion in the brain, brainstem,
cerebellum, or upper cervical spine (Figure 237.1 A-D).
Hemiplegia, whether spastic or flaccid, always implies a central nervous
system lesion. Hemiplegia does not occur as a consequence of peripheral
nervous system involvement.
Unilateral lesions in the cortex,
centrum semiovale, internal capsule, midbrain, and upper pons may produce
contralateral hemiparesis and facial weakness (Figure 237.1 A).
Unilateral cerebellar lesions
may produce hemiparesis on the same side of the cerebellar lesion. The
face is usually spared.
Unilateral pontine lesions produce
contralateral hemiparesis and ipsilateral facial weakness (Figure 237.1
B). The involvement of the
face on the same side as the lesion (on the side opposite from the hemiparesis)
is due to direct damage to the facial motor nucleus or its fibers. The
sparing of the face on the side of the hemiparesis occurs because the
fibers innervating the facial musculature of the opposite side cross to
the other side at the level of the midbrain. The presence of contralateral
hemiparesis and ipsilateral facial weakness is referred to as cross-hemiplegia.
Unilateral upper medullary lesions
may produce contralateral hemiplegia with sparing of the facial musculature
(Figure 237.1 C). Medullary lesions more commonly produce paraparesis.
They damage the pyramidal system as their left and right fibers cross
to the other side.
unilateral upper spinal cord lesion may produce ipsilateral hemiplegia
with sparing of the facial musculature (Figure 237.1 D).
Possible sites of anatomical injury producing hemiparesis. A: brain and
midbrain; B: upper pons; C: lower pons and medulla; D: rostral spinal
cervical cord; V: ventricles; T: thalamus; FN: facial nerve; UQ: upper
quadrant; LQ: lower quadrant; BP: brachial plexus; LSP: lumbosacral plexus.
The colored rectangles indicate the location of weakness produced by damage
to the various components of the somatic motor system.